Some protocols are routable and others are nonroutable. Nonroutable protocols are typically broadcast-based, which adds overhead and traffic to a network. Routers serve as traffic forwarders for remote end systems. Routing protocols and mechanisms are needed to build and maintain route information. To forward traffic, routers need to know the destination address, from which source it can learn the path to a given destination, the best path, and a way of verifying the most current path to take.

Nonroutable Versus Routable Protocols

Some protocols can be routed and others cannot. Nonroutable protocols, such
as DEC's LAT (Local Area Transport) and NetBEUI (Network Basic Input/Output
System), do not contain network layer addressing information. Without the
network layer addressing information, routers lack the information they need to
determine how to decide for which network or host traffic is destined.
Therefore, these protocols need to be bridged or switched using layer 2
information such as a MAC address.

As a general rule nonroutable protocols are broadcast-based, which adds
overhead and traffic to a network. For this reason, you usually install a router
to isolate the traffic from propagating through an entire internetwork.

However, on some occasions, you may find it necessary to allow this traffic
to be forwarded to other segments. You can accomplish this by replacing the
router with a layer 2 device (like a bridge or switch) or by configuring the
intervening router to forward the nonroutable traffic without determining the
specific route this traffic will take.

Note

Routers configured to bridge traffic are referred to as Brouters, allowing
the router to bridge traffic using the layer 2 address like a bridge. Routers
can transparently bridge, source route, or combine the two methods, depending on
your needs and the architectures supported, such as Ethernet or Token-Ring. How
a router bridges traffic is determined by the layer 2 implementation you have in
place.

Routable protocols are protocols that can be forwarded by a router. Any
protocol that has layer 3 logical network addressing can be routed, like IP,
IPX, and AppleTalk. Because IP, IPX, and AppleTalk all supply the network layer
addressing information and any protocol, application or service that runs on top
of these protocols may also be routed. Routers forward traffic based on network
layer logical destination addresses; without it routing would not be possible.
We discussed IP and logical network layer IP addresses in Chapter 2, "IP
and IP Addressing."